text: Glycine is biosynthesized from serine, catalyzed by serine hydroxymethyltransferase (SHMT). The enzyme effectively replaces a hydroxymethyl group with a hydrogen atom.
  SHMT is coded by the gene glyA. The regulation of glyA is complex and is known to incorporate serine, glycine, methionine, purines, thymine, and folates,  The full mechanism has yet to be elucidated. The methionine gene product MetR and the methionine intermediate homocysteine are known to positively regulate glyA. Homocysteine is a coactivator of glyA and must act in concert with MetR. On the other hand, PurR, a protein which plays a role in purine synthesis and S-adeno-sylmethionine are known to down regulate glyA. PurR binds directly to the control region of glyA and effectively turns the gene off so that glycine will not be produced by the bacterium.